1. Field of the Invention
This invention relates to the field of scanner apparatus and methods; and more particularly to inspection systems that scan luggage and cargo to detect residues of explosives or other contraband materials.
2. Description of the Prior Art
In recent years, the prevalence of criminal activity that entails transportation of weapons and contraband materials has been a significant public concern. It has thus become vital to develop systems for detecting the presence of these materials, both when shipped as luggage or cargo and when carried by an individual. Of particular concern is the need to detect items used as weapons by terrorists, including ordinary firearms and knives, items such as explosive or incendiary substances, and materials which present biological, chemical or radiological hazards to people and property. The detection of illicit drugs and narcotics being transported is also of concern.
The detection of contraband in the context of air and rail transportation is especially challenging, given the need to examine large numbers of people and articles of luggage and cargo within acceptable limits on throughput and intrusiveness. Although physical inspection is a widely practiced and important technique, it is slow, cumbersome, labor intensive, and dependent on the alertness and vigilance of the inspector.
Automated systems that screen for contraband have been sought for many years. Various techniques have been proposed to detect contraband objects and materials either directly or indirectly. Magnetometry is widely used, and is sometimes effective in detecting metallic objects carried by persons, but is not suited for screening cargo, which legitimately may contain large amounts of metal. Nuclear techniques, including x-ray, gamma-ray, neutron activation, and nuclear magnetic resonance methods, are applicable for screening inanimate objects, but pose risks that generally preclude their use for screening humans. In some cases, they are able to detect metallic objects, including weapons and ancillary devices such as wires, power supplies, batteries, and triggering mechanisms for explosive devices. However, there increasingly exist threats posed by explosives associated with largely non-metallic objects, which the aforementioned methods are less able to detect. The advent of modern plastic explosives presents an especially significant threat. Even a modest, readily concealable amount of these substances can cause a substantial explosion. Moreover, miscreants have become increasingly adept at disguising weapons and explosive devices as ordinary, innocuous objects. As a result, more refined indirect methods for detection of explosives are urgently sought.
Many of the indirect methods rely on the presence of vapor emanating from suspect material. One such indirect method, widely used in law enforcement, employs dogs trained to sniff preferentially for explosives, drugs, and the like. The remarkable olfactory sensitivity of dogs has been known and exploited for centuries. However, they are subject to fatigue, behavior variations, and the need for careful handling, training, and reinforcement from their masters. It therefore remains highly desirable to have luggage scanning systems that are not subject to these limitations. Also needed are luggage and cargo scanning systems that rapidly and accurately discriminate among different substances and indicate the quantity and location of a critical substance.
The task of indirectly detecting the presence of suspect materials is further complicated by their wide variability in vapor pressure. Some explosives, including nitroglycerin (NG), dynamite, EGDN, and EGTN, are comparatively volatile, exhibiting significant vapor pressure at room temperature. DNT and TNT have lower, but still appreciable room-temperature vapor pressure. However, some of the most critical materials for which detection is sought, e.g. drugs, such as cocaine and heroin, and plastic explosives, such as SEMTEX and C-4, are far less volatile, having room temperature vapor pressures as much as ten million times lower. It is virtually impossible to detect vapor naturally emanating from these low volatility materials. They are even more difficult to detect if sealed inside luggage or packaging.
It is known that certain contraband materials for which detection is sought are inherently sticky. This characteristic is a notable property of many plastic explosives. As a result, particulate residues are likely to be present (i) on the hands of a person who has even casually handled the contraband, even after repeated hand washing, (ii) in fingerprints on surfaces such a person has subsequently touched, and (iii) as cross-contamination on the surface of a vehicle, shipping container, or luggage in which the material has been placed. For example, a measurable amount of ammonium nitrate (AN) residue has been found on the lease documents for rental trucks; and significant amounts of the explosives PETN (pentaerythritol tetranitrate) and/or AN have also been found on clothing and inside vehicles of suspects in two well-publicized bombings. Therefore, explosive residue will likely persist in large amounts on the explosive packaging and its environs, as well as on the individuals involved in building, handling, and transporting the explosive device, thereby providing an avenue for detection of the presence of explosives. The detection of even trace residues of critical substances on a person, article, or vehicle suggests a strong likelihood of association with illicit activity warranting further investigation.
The dual challenges of sample collection and analysis continue to impede development of satisfactory screening systems for the aforesaid contraband materials. As previously described, many of the materials whose detection is most critical have extremely low vapor pressure. The equilibrium concentration in the atmosphere near a contaminated fingerprint may be only parts per billion or trillion, a value too low for known detection schemes. Hence, previous detection methods have frequently employed mechanical means for collecting and/or concentrating a sample to achieve detectability. In some cases, disposable swabs or wipes of dry paper or cloth are rubbed by an operator against luggage or shipping containers to pick up detectable amounts, if any, of particulate residue. Such wipes may also be wetted with a solvent to facilitate residue pickup. In either case, the wipe is subsequently transferred to a suitable detection system for chemical analysis.
If carried out with rigorous attention to collection protocols, wipe techniques provide an effective method of manually collecting samples from the surface of objects. However, known wipe systems have a number of significant limitations. They generally require an operator and are not conveniently adapted to automation. Their throughput is limited by the cumulative time needed for the essential multiple operations—in addition to the actual analytical time, the process requires the prior intermediate steps of wiping the article under test and transferring the wipe to the detection system. The detection efficacy and success of wipe systems is generally dependent on human factors. Stress and the frequent confusion extant in a busy public facility may cause an operator to fail to carry out an adequate sampling. The wiping operation frequently fails to cover a sufficiently representative portion of an article to insure that whatever residues are present are actually captured. Lint, dirt, solvent, and other extraneous material of no interest are inevitably introduced into the detection system. In some cases these contaminants reduce the system's sensitivity by diluting the concentration of the analyte and necessitating frequent, non-productive cleaning operations.
Other known systems have employed mechanical brushing or shaking of articles or impingement of a compressed gas stream to dislodge residue particles. While these methods are more amenable to automation than wiping-based methods, they still are not sufficiently fast and efficacious for the demanding requirements of inspecting items to be carried as cargo or hand luggage on aircraft, for example. Furthermore, regulation of the pressure and volume of the gas stream is a significant challenge, as the flow must be sufficient to dislodge particles but not so high that it is not possible to capture what is removed.
Systems have also been proposed for detecting the presence of residues on a human subject passing through a tunnel-like portal. The portal may include means for flowing gas across the subject to dislodge particulate residues, collecting the gas, filtering or otherwise concentrating the particulates to above a detection limit, and passing the concentrated sample to a suitable detector. However, improvement in these systems is still desired. Flowing gas is at best an inefficient vehicle for collecting adequate sample. Disruptions of the airflow owing to the motion of subjects passing through the portal further compromise sample collection. In addition, the need to pre-concentrate a sample limits the analysis rate, making it difficult to reliably associate detection of contraband substances of interest with a specific person passing through the sampling portal.
Each of the indirect screening systems previously discussed requires means for sample collection and analysis that discriminate suspect substances from components normally present in the atmosphere. To be effective, the sample collection and analysis means must additionally discriminate suspect substances from the myriad of vapors produced by items of ordinary commerce.
A number of vapor detection methods have been proposed. These vapor detection methods have found use in laboratory analysis. Among them are electron capture detection, gas chromatography detection, mass spectrometry detection, plasma chromatography detection, bio-sensor detection and laser photoacoustic detection.
There have also been suggested systems for detecting explosive residues that do not rely on vapor detection. One example is the use of a controlled burst of laser radiation to induce selective deflagration or micro-detonations of explosive residues on the surface of an article. The resulting reaction produces an optical signature characteristic of the explosive residue. The system relies on detection of this optical signature. As used herein, the term “deflagration” means a slow chemical oxidation of the material, with a burn front which propagates at less than the velocity of sound. The term “detonation” as used herein means a reaction similar to deflagration that occurs at a much faster rate. Detonation is characterized by wave propagation at a supersonic rate with respect to the unreacted material.
Notwithstanding the aforementioned schemes both for sample collection and analysis, there remains a need in the art for integrated systems capable of reliably, accurately, and rapidly detecting the presence of contraband substances, especially explosives, accelerants, and illicit drugs. More particularly, there is need for systems that are readily automated for semi-continuous or continuous inspection and detection of the presence of residues of such materials on luggage, cargo, vehicles, freight containers, and related items. Such systems are highly sought, especially in the context of airport screening, but would be equally valuable for courthouses, stadiums, schools, government offices, military installations, correctional institutions, and other public venues that might be targets of terrorist or similar criminal activity.